1. Field of the Invention
The present invention generally relates to digital communications systems and, more particularly, to a digital modulator which can efficiently produce a non-integer number of samples per symbol time. The invention has particular application in digital cellular telecommunications which employ frequency shift keying and quadrature phase shift keying at different rates.
2. Description of the Prior Art
In digital voice communication systems, such as cellular telecommunications, input sample rates can be different from output sampling rates, resulting in a non-integer relationship between the number of input and output samples. As a specific example, in the current U.S. analog cellular telephone standard, the control channel uses a Frequency Shift Keying (FSK) symbol rate of 10 KHz. The new U.S. digital cellular telephone standard EIA/IS-54A (Electronic Industries Association/Interim Standard) adds control signaling using 24.3 K-Symbol/Sec. QPSK (Quadrature Phase Shift Keying) modulation. In this case, it is desired that the same modulator which generates the 10 K-Symbol/sec. FSK modulation also generates the 24.3 K-Symbol/sec. QPSK modulation. This works out to be 2.43 QPSK samples per FSK symbol time.
There are two traditional approaches which could be used to synthesize a modulation waveform which has a non-integer number of samples per symbol time. The first approach would run a Direct Digital Synthesizer (DDS) at a much higher clock rate and then decimate its output to achieve the desired number of samples per symbol time. Another approach would be to generate the modulation with an integer number of samples per symbol time and then interpolate/decimate using appropriate filters to the desired sampling rate.